Process for producing thermoplastic resin foams by adding a blowing agent and other necessary additives to a thermoplastic resin, mixing the respective components and extrusion foaming the mixture, as well as apparatus for implementing the processes are widely known. Among those processes, the approach of using an extruder having a rotating screw in a barrel finds extensive commercial use because of its capability for continuous production of desired foams in large volumes. This approach typically comprises the following steps: heating a thermoplastic resin to melt in an extruder with a blowing agent and other necessary additives are forced into the melt; mixing the respective components uniformly under pressure; cooling the mixture to a temperature suitable for expansion; extruding the cooled mixture through a specified die; and allowing the molded part to expand in the reduced atmosphere thereby producing desired foams in a continuous manner.
For uniform expansion, it is important that the blowing agent be uniformly mixed with the thermoplastic resin and that the molten resin be uniformly cooled to the temperature suitable for expansion. To meet these needs, various methods and apparatus have been proposed that are capable of both uniform mixing of the resin feed with the blowing agent and uniform cooling of the molten resin composition. One of these proposals is described in JP-B-54-42026 (the term "JP-B" as used herein means an "examined Japanese patent publication") and it uses a cooler that has a rotating shaft in a barrel that can be cooled; the shaft has a plurality of vanes or blades in a plate form that are provided on the circumference in a staggered pattern in both an axial and a circumferential direction, with the length of each vane being parallel to the axis of the shaft. Each vane has a plurality of holes that extend through the plate thickness. The through-holes in adjacent vanes are oriented in two different directions, one being from the inner to the outer circumference and the other being vice versa. According to JP-B-54-42026, this arrangement enables the molten foamable resin to be mixed and cooled uniformly. However, the cooler under consideration has no ability to thrust the resin and in order to prevent the loss of extrusion force, the diameter of the rotating shaft has to be increased to such an extent that the resulting equipment is difficult to install adjacent the extruder and adjust for proper operation.
Another proposal is made in JP-B-60-52926 (corresponding to U.S. Pat. No. 4,454,087) and it consists of providing a cooler of the above-described type and a zigzag mixer which is a kind of static mixers. A problem with this proposal is that a loss in the extrusion force occurs in the zigzag mixer and in order to compensate for the resulting loss in extrusion output, a sufficient thrust force must be secured by installing two extruders behind the cooler but then the overall production equipment becomes complex.